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Certain Pb-Sn solder alloys are eutectic. Since a mechanical mixing takes place in eutectic alloys, tin and lead are clearly distinguishable on a microscopic image. In some cases, however, two metals (A and B) mix so thoroughly that their structures disappear and a new common structure arises. If, for example, alcohol is dissolved in water, it is completely and homogeneously mixed in it, making the two components indistinguishable. It is impossible to remove some water if the whiskey is too strongly diluted – you have to pour more whiskey in it instead. A good example for solid solutions of metals is the variety of nickel -copper alloys (constantan). In general, substances with the same lattice structure form solid solutions much easier. In Sn-Pb alloys, tin dissolves in lead with a mass proportion of 25% at the eutectic temperature. Each time the mixture is cooled down and heated again tin is precipitated or goes back into solution. Therefore, in order to determine the characteristics of a solder alloy, it is necessary to allow the sample to rest for several hours or days.
The tendency to enter into solid solution is the explanation for the phenomenon that two metals of the same lattice type exhibit strong abrasion when rubbed together.